I apologize for the dirty mirror. They always look so clean until you hit them with a flash. At least I don't have all my bathroom clutter in the frame, too :D
1 conveniently flat box
1 rubber band (to attach box to flash)
spray glue (to stick the foil to the inside of the box)
packing tape (to stick everything else together)
Works way better than the old index card, and I can use it for portrait photography And to think I was about to pay $14.99 +s/h for one when I've got all these boxes laying around from all the crap I buy online.